This application claims the priority of Japan patent application Serial No. P11-355522 filed on Dec. 15, 1999 and Japan patent application Serial No. P2000-154687 filed on May 25, 2000.
1. Field of the Invention
The present invention relates to a photomask that is used for producing high density integrated circuits like LSI and VLSI or for forming other microscopic patterns, and to a blank for producing the such a photomask, especially relates to a halftone phase shift photomask by which projection image in fine dimension can be obtained and to a blank for producing the halftone phase shift photomask.
2. Description of the Related Art
Semiconductor integrated circuits, including IC, LSI and VLSI, are produced by repeating the lithography process of using a photomask, especially in case of forming fine circuit patterns, it is studied to use phase shift photomasks disclosed in, for example, Japanese Patent Application Laid-Open No. 58-173744, Japanese Patent Application Publication No. 62-59296 and others.
Although phase shift photomasks with various types of constitution are pro posed, among them, for example, halftone phase shift photomasks disclosed in Japanese Patent Application Laid-Open No. 4-136854, U.S. Pat. No. 4,890,309 and others attract attentions from the viewpoint of possibility of being put to practical use in early time.
As for halftone phase shift photomasks, in literatures, including, for example, Japanese Patent Application Laid-Open Nos. 5-2259 and 5-127361, the improvement of yield rates by reducing the number of production processes, the constitution with possibility of reducing the cost, preferable materials and others have been proposed.
In the following, a common halftone phase shift method and a common halftone phase shift photomask will be briefly explained with reference to the accompanying drawings.
FIG. 14(a) to FIG. 14(d) are views showing the principle of a halftone phase shift method (a halftone phase shift photo lithography), and FIG. 15(a) to FIG. 15(d) are views showing the principle of a photomasking method using a photomask except a phase shift photomask. FIG. 14(a) and FIG. 15(a) are sectional views of a photomask, FIG. 14(b) and FIG. 15(b) show the amplitude of light on a photomask, FIG. 14(c) and FIG. 15(c) show the amplitude of light on a wafer, and FIG. 14(d) and FIG. 15(d) show the strength of light on a wafer. Reference numerals 911 and 921 denote substrates, 912 denotes a halftone phase shift film in which the phase of incident light is substantially shifted by 180 degree and the transmittance of transmitted light is within the range of 1 to 50%, 922 denotes a 100% light shielding film, and 913 and 923 denote incident light.
In a conventional photomasking method, as shown in FIG. 15(a), the 100% light shielding film 922 made of chromium or the like is formed on the substrate 921 consisting of quartz glass to form a light transmission part (an aperture) in a desired pattern. In this case, the distribution of light strength on a wafer is broadened toward the end as shown in FIG. 15(d), resulting in inferior resolution.
On the other hand, in a halftone phase shift method, because the phase of light transmitted through the halftone phase shift film 912 is substantially inverted to that of light transmitted through the aperture, light strength on boundary parts of patterns on a wafer becomes zero as shown in FIG. 14(d), which can prevent light from broadening toward the end. In this case, therefore, resolution can be improved.
Here, it should be noted that, in phase shift photo lithography that belongs in types except a halftone phase shift method, because a light shielding film and a phase shifter film are formed in different patterns with different materials, the plate making process is needed to be repeated at least 2 times, while because it is enough to form only one pattern in the halftone phase shift photo lithography, it is essentially needed to carry out the plate making process only once and this is a big advantage in halftone phase shift lithography.
In the halftone phase shift film 912 of a halftone phase shift photomask, two functions, that is, phase inversion and permeability control are needed. Out of them, as for the phase inversion function, it is sufficient that phase will be substantially inverted between exposure light transmitting through the halftone phase shift film 912 and exposure light transmitting through the aperture. Here, if the halftone phase shift film 912 is considered as an absorption film that is shown, for example, in pages 628 to 632 of xe2x80x9cPrinciples of Opticsxe2x80x9d written by M. Born and E. Wolf, since multiplex interference can be ignored, phase change xc3x6of vertically transmitted light will be calculated using the following expression. And when the value of xcfx86 is within the range of nxcfx80xc2x1xcfx80/3 (n is an odd number), the above-mentioned phase shift effect will be obtained.                     φ        =                                            ∑                              k                =                1                                            m                -                1                                      ⁢                          xe2x80x83                        ⁢                          x                              k                ,                                  k                  +                  1                                                              +                                    ∑                              k                =                2                                            m                -                1                                      ⁢                          xe2x80x83                        ⁢                          2              ⁢                              π                ⁡                                  (                                                            u                      k                                        -                    1                                    )                                            ⁢                                                d                  k                                /                λ                                                                        Expression  (1)            
Further, in expression (1), xcfx86 is a phase change caused to light vertically transmitting through a photomask in which a multilayer film of (mxe2x88x922) layers is formed on the substrate, "khgr"k,k+1 is a phase change occurring in the interface between a kth layer and a (k+1)th layer, uk and dk are the refractive index and film thickness of the kth layer, respectively and xcex is the wavelength of exposure light, providing that the layer of k=1 is the above mentioned transparent substrate and the layer of k=m is air.
On the other hand, the transmittance of exposure light transmitted through the halftone phase shift film 912 for obtaining a halftone phase shift effect is determined by the dimension, area, arrangement, shape and the like of a transcription pattern, and differs depending on patterns.
In order to substantially obtain the above-mentioned effect, the transmittance of exposure light transmitted through the halftone phase shift film 912 should be within the range of the optimum transmittancexc2x1some percents, where the center value is the optimum transmittance determined by the pattern.
Generally, this optimum transmittance greatly varies within the wide range of 1 to 50% depending on transcription patterns when the transmittance in the aperture of the halftone phase shift film is set to 100%. That is, in order to adapt to all patterns, halftone phase shift photomasks having various transmittances are needed.
In a practical situation, the phase inversion function and the transmittance control function are determined by a complex refractive index (a refractive index and an extinction coefficient) and film thickness of a material forming the halftone phase shift film. In case of a multilayer structure, the phase inversion function and the transmittance control function are determined depending on a complex refractive index and a film thickness of each layer. In other words, it is possible to use a material adjustable its film thickness so as to control phase difference calculated by the above mentioned expression (1) within the range of nxcfx80xc2x1xcfx80/3 (n is an odd number) as a halftone phase shift film of a halftone phase shift photomask.
As thin film materials for photomask patterns, tantalum based materials are commonly known as shown in, for example, Japanese Patent Application Laid-Open No. 57-64739, Japanese Patent Application Publication Nos. 62-51460 and 62-51461. Because tantalum based materials are extremely excellent in processing properties, chemical stability after being processed, and others, they have been vigorously studied and tried to apply them in halftone phase shift films by oxidizing ornitriding tantalum as disclosed in, for example, Japanese Patent Application Laid-Open Nos. 5-257264, 7-134396 and 7-281414. Furthermore, with the advancement of shortening the wavelength of exposure light in connection with the minuteness of LSI patterns, studies have also been carried out to use materials of tantalum silicides that are able be applied to exposure light of shorter wavelength as shown in, for example, Japanese Patent Application Laid-Open No. 6-83027.
However, generally, dry etching of tantalum silicides is carried out using an etching gas of fluorinated compounds, including CF4, CHF3, SF6, C2F6, NF3, CF4+H2 and CBrF3, but there was such a problem that in this case, the fluorinated-etching gas also etches a transparent substrate of synthetic quartz and the like, and therefore, dry etching with high precision can not be carried out. Generally, concerning the production of a halftone phase shift photomask, the high precision control of the phase angle is indispensable, but if a quartz substrate is also etched when etching a halftone phase shift film as mentioned above, errors will be induced in phase difference by etching depth. Furthermore, because etching of a halftone phase shift film has an important role in controlling pattern dimensions, it is desired to set conditions so that the uniformity and reproducibility of pattern dimensions can be obtained as good as possible, but there may be such a problem that a margin in the range for setting conditions becomes narrow due to the addition of a new parameter of an etching selective ratio to quartz. This problem will be induced because the optimum etching process for attaching much importance to dimension control does not always accord with that for attaching much importance to phase difference control. That is, materials of tantalum silicides for a halftone phase shift film in themselves show excellent processing properties and chemical stability after being processed, but when phase difference control in high degree is further taken into consideration, the patterning with high precision will be difficult.
An object of the present invention is to provide a halftone phase shift photomask capable of providing an improved etching selective ratio to substrate materials such as a quartz substrate, as well as maintaining desirable properties of tantalum silicides including applicability to exposure light with the short wavelength, excellent processing properties, and chemical stability after processing.
Another object of the present invention is to provide blanks for a halftone phase shift photomask that permit to form halftone phase shift photomasks with such excellent properties.
Another object of the present invention is to provide a halftone phase shift photomask capable of providing an etching selective ratio to substrate materials such as a quartz substrate whether a material of tantalum silicides is used or not.
A halftone phase shift photomask to be provided in the present invention comprises a transparent substrate and a halftone phase shift film provided on said transparent substrate, characterized in that said halftone phase shift film has a multilayer construction in which at least a first layer capable of being etched with a chlorinated gas and a second layer capable of being etched with a fluorinated gas are disposed in this order from the side near said transparent substrate, and that said halftone phase shift film also comprises apertures made by removing part of said halftone phase shift film in a prescribed pattern.
The present invention also provides a blank enabling to produce such a halftone phase shift photomask. The blank for a halftone phase shift photomask comprises a transparent substrate and a halftone phase shift film provided on said transparent substrate, characterized in that said halftone phase shift film has a multilayer construction in which at least a first layer capable of being etched with a chlorinated gas and a second layer capable of being etched with a fluorinated gas are disposed in this order from the side near to said transparent substrate.
When the blank having the halftone phase shift film with such a multilayer construction is first etched with a fluorinated gas, the second layer of the halftone phase shift film is patterned in a prescribed shape. Next, the halftone phase shift film of the blank is etched with a chlorinated gas in the pattern coincident with that etched with the fluorinated gas, and thus the first layer of the halftone phase shift film is patterned coincidentally with the second layer, but the transparent substrate is not substantially etched. As a result, only the halftone phase shift film can be precisely etched. And it is also possible to control freely the phase angle and transmittance by forming 2 or more layers constituting a halftone phase shift film with respective different materials and by controlling the thickness of each layer.
Because the phase angle and transmittance of a halftone phase shift photomask can be controlled in high precision as well as improving an etching selective ratio to the transparent substrate, it becomes possible to obtain a projection image with precise minute dimensions by using the halftone phase shift photomask.
As a method of improving the etching selective ratio to the substrate, it is known to provide an etching stopper layer between the substrate and the halftone phase shift film. By this known method, however, the etching stopper layer will remain in the aperture of the accomplished halftone phase shift photomask to affect the phase angle and light transmittance in the aperture. To the contrary, any no etching stopper layer remains at the aperture in the present invention.
The first layer of the halftone phase shift film is disposed at a portion near the transparent substrate, and it is formed of a material capable of being etched with a chlorinated gas. For example, the material capable of being etched with a chlorinated gas may be selected from among tantalum based materials and chromium based materials, and the first layer can be formed of the thus selected material.
The second layer of the halftone phase shift film is disposed at a portion distant from the transparent substrate in comparison with the first layer. It is preferable that the second layer is formed of tantalum silicide based material. Though the tantalum silicide based material does not have so large etching selective ratio to the transparent substrate, it is excellent in processing properties, chemical stability and applicability to a light exposure with a short wavelength. Accordingly, when a halftone phase shift layer made of the tantalum silicide based material (namely, the second layer) is formed on the transparent substrate via the other halftone phase shift layer having a large etching selective ratio to the transparent substrate (namely, the first layer), it is possible to use the layer made of the tantalum silicide based material with a large etching selective ratio.